1. Field of the Invention
This invention relates in general to fluorescent lighting luminaries and more particularly to a new operating mode of fluorescent tubes in a luminary.
2. Description of the Prior Art
A fluorescent tube is a discharge glass bulb whose inner side is covered with fluorescent layer that reacts by emitting visible light when excited by ultraviolet radiation sourced from the gas medium that fills the bulb. Such a gas contains very low pressure mercury vapor.
FIG. 1 is a diagram describing the principle of construction and operation of a single fluorescent tube luminary. A flux of electrons 4, crossing the tube 14 between the two electrodes 16 placed at each end, excites mercury atoms 3 and produces ultraviolet radiation 5. These electrodes consist of electron thermo-emissive filaments (called pre heating Cathodes) that must heat to incandescent. The alternative main current voltage is used to supply power to the cathodes through a high inductive coil (called Ballast) included to the circuit, that limits the current drawn by the low impedance ionized gas to an acceptable value.
Igniting the conduction through the gas requires a special device (called Starter) connecting the heating filaments directly one to the other, in parallel with the gas medium (see FIG. 1, low section). The starter may consist of a filament bulb including a switch reacting to temperature opened at normal temperature. When the power is applied to the circuit, the current starter bulb lights on and internal temperature goes up rapidly while the cathodes of the fluorescent tube are incandescent. When the temperature is high enough, the switch closes, short cutting the starter which rapidly decreases in temperature and thus opens again the switch. This sudden interruption causes the current to induce a high voltage surge at the coil's outputs (self induction effect) thereby producing an igniting conduction between the two cathodes, through the mercury vapor heated by the incandescent filaments. From this point, the starter is not activated as long as the conduction of the vapor remains. The filaments of the cathodes stay incandescent due to their construction and position that drive a part of the current crossing the tube to flow through their surface also hit by mercury ions that help to maintain temperature by dissipating collision energy.
As conduction is ignited and current stabilizes, the tube's impedance decreases significantly. Due to its value of impedance at operating frequency of the main power, the ballast coil ensures proper current limitation. Such a system is defined as “magneto-inductive ballast”.
However, some evolutions exist in ballast technology that improve the simple description above, and stated in FIG. 1.
Generally, a ballast is serial impedance that stabilizes the current in the fluorescent tube, usually, as mentioned above, simple inductors are used as ballasts because they operate as reactances with small losses when serial coupled to the tube. Some magnetic ballasts provide more features than serial impedance for the tube like for instance transformers for increasing voltage levels.
In the goal of energy saving, other types of ballast have been developed applying solutions that use semiconductors. This more sophisticated design brought also possibilities to use operating frequencies higher than conventional 50/60 Hz from main electric supply. Frequency in the range of 25 kHz has been employed. Examples of electronic Ballast design are described in patents WO 00/21342 published April 2000, WO 99/05889 published February 1999, WO 97/33454 published September 1997, WO 99/60825 published November 1999, WO 98/34438 published August 1998 and EP-O-955794-A2 published November 1999. Exposed solutions relate mainly to current savings and life time improvement of fluorescent tubes by optimizing different parameters such as waveform, voltage amplitude . . . etc.
U.S. Pat. No. 6,262,542 discloses an electronic Ballast including a lamp driving circuit having a pulse width modulated signal generator to control the duty cycle of the square wave form current flowing through the lamp. The '542 patent further describes a control signal that is included into the circuitry for monitoring the lamp operation. It is also to note that lamp coupling method as described in U.S. Pat. No. 6,262,542 forces the current to flow through the cathode's filaments.
U.S. Pat. No. 4,902,939 discloses a driving circuitry dedicated to avoid light flickering when witching on and off the power from minimum to maximum variable lighting intensity. Obviously the objective is not to increase the power efficiency of fluorescent lamps. The major difference with the invention is that the driving voltage described in U.S. Pat. No. 4,902,939 consists of sinusoidal waveforms directly derived from main power supply.
Although existing electronic ballasts intend to bring energy savings through their operating modes of fluorescent tubes as well as to extend life time of fluorescent lamps, considerable research and development remain to be done in this domain. The invention discloses a particular brand new operating mode for fluorescent tubes that reduces current drawn by the lamp for 40% to 50% regarding to conventional magnetic ballasts mostly installed in fluorescent lighting fixtures.
In addition, life time of tubes driven by the invention increases for up to 3 times and light emitted does not flicker or suffer stroboscopic effect.
Above benefits are obtained by operating fluorescent luminaries in a way disclosed in the present invention. Such luminaries include one or more standard fluorescent tubes that contain mercury vapor gas and heating filament cathodes at both ends, a fixture that integers proper holding and connection devices for the fluorescent tubes, and one ballast for driving the fluorescent tubes. Ballast operating mode differs from existing systems by the fact that it uses voltage pulses applied to the electrodes for exciting the fluorescent gas, such pulses consisting of non periodic voltage levels separated by variable duration dead times.